Precast concrete curb inlets have been made for a long time. One shown in U.S. Pat. No. 4,192,625, included an integral form portion, which was described as preferably being made from fiberglass. A similar curb inlet, made entirely from reinforced concrete without a fiberglass form portion, had been on the market long before that patent issued.
In both of these curb inlets, a portion extending across the front and spaced down from the top-panel providing the inlet opening was permanently in place before installation and remaining there after installation, it was used as a form when the gutter was poured, and it could not be used again. Therefore, each time a curb inlet of that type was set in place and the gutter was poured the gutter form member remained with the curb inlet.